There currently is substantial interest in tonometric estimation of gastrointestinal intramucosal pH (pH.sub.i) as a means for monitoring mesenteric perfusion in critically ill patients. There are two reasons for this: First, alterations in mesenteric perfusion have been associated with derangements in gut barrier function. It has been hypothesized that such derangements might permit the systemic absorption of intact microbes or microbial products into mesenteric lymphatics and the portal venous system, thereby triggering or sustaining the release of pro-inflammatory mediators implicated in the pathogenesis of the multiple organ dysfunction syndrome. Second, studies suggest that blood flow is diverted away from the splanchnic bed in animals subjected to hemorrhage, sepsis, sterile peritonitis, or heart failure. Thus, gastrointestinal pH.sub.i may be able to serve as a sentinel marker of tissue hypoperfusion in various shock states.
Tonometric estimation of mucosal gas tensions has been performed in the urinary bladder and gallbladder. This concept was extended to the gastrointestinal tract when it was observed that the partial pressure of CO.sub.2 in the mucosa could be estimated by placing liquid in an isolated bowel segment and allowing adequate time for equilibration with tissue CO.sub.2. The application of tonometry as a practical means for monitoring patients has been developed, based on the observation that saline in a permeable silicone balloon equilibrates with tissue pCO.sub.2. It has been proposed that tonometry be used to indirectly estimate pH.sub.i in the stomach or colon by inserting the tonometrically measured pCO.sub.2 (TpCO.sub. 2) into a modified Henderson-Hasselbach equation, using the assumption that intracellular and arterial (HCO.sub.3) are equal, and, then backcalculating pH. The notion that tonometry could be used to estimate pH.sub.i has been validated by showing the qualitative agreement between values for this parameter obtained by direct measurement with those simultaneously made using the prior art tonometric methods. Further improvement in the accuracy of gastric tonometry as a clinical tool resulted from the observation that intraluminal CO.sub.2 production from the combining of gastric acid and secreted bicarbonate could be obviated by pretreatment with H.sub.2 antagonists. Other techniques to measure splanchnic pCO.sub.2 have included mass spectrometry of pCO.sub.2 dissolved in intraluminal fluid and Severinghaus electrode analysis of gastric fluid aspirates, obtained 30 minutes after installation of 30 mL of saline via a nasogastric tube.
Several studies in animals have confirmed the ability of tonometry to provide an early signal of gut ischemia in porcine models of hemorrhagic and septic shock and to indicate loss of ileal mucosal barrier integrity. Clinical investigations in critically ill patients also have documented the value of tonometric pH.sub.1 assessment as a means for predicting massive bleeding from stress ulceration, mortality in surgical intensive care unit patients, intraoperative assessment of splanchnic hypoperfusion, and prediction of complications in cardiac surgery patients. In a prospective trial, survival was greater in critically ill patients whose therapy was guided by the pCO.sub.2 in samples of gastric fluid.
The implementation of the prior art tonometric techniques, however, requires relatively costly equipment, for example, a special nasogastric tube fitted with a silicone balloon. Furthermore, the relatively large diameter of the prior art equipment also has prevented its use in neonatal and pediatric patients, where alternative means of monitoring perfusion, such as Swan-Ganz catheterization, are impractical and seldom used. Also, the prior art techniques are relatively slow in response, generally precluding real-time monitoring. Also, the prior art techniques generally require transport of extracted samples to an analysis site, resulting in increase in costs due to transport, as well as increase in risk of sample contamination or loss during such handling. Moreover, correction for temperature of the gastrointestinal lumen is also required; the latter is inherently difficult to accomplish in a normal clinical setting.
It is an object of the present invention to provide an improved tonometric method and apparatus for measuring gastrointestinal intraluminal pCO.sub.2 and optionally pO.sub.2.
Another object is to provide a method and apparatus for continuously monitoring intraluminal gastrointestinal pCO.sub.2 and optionally pO.sub.2.
Yet another object is to provide a method and apparatus for measuring intraluminal gastrointestinal pCO.sub.2 and optionally pO.sub.2 in small patients, where size restricts the use of conventional tonometry.